Protein tyrosine phosphorylation plays an essential role in the regulation of cell growth, proliferation and differentiation (reviewed in Hunter, T. (1987) Cell 50:823-8291). This dynamic process is modulated by the counterbalancing activities of protein tyrosine kinases (PTKs) and protein tyrosine phophatases (PTPs). The recent elucidation of intracellular signaling pathways has revealed important roles for PTKS. Conserved domains like the Src homology 2 (SH2) (Suh, P.-G., et al., (1988) Proc. Natl. Acad. Sci. (USA) 85:5419-5423) and the Src homology 3 (SH3) (Mayer, B. J., et al., (1988) Nature 352:272-275) domains have been found to determine the interaction between activated PTKs and signal transducing molecules (reviewed in Pawson, T., and Schiessinger, J. (1993) Current Biol. 3:434-442; Koch, C. A., et al., (1991) Science 252:668-674). The overall effect of such protein interactions is the formation of signaling cascades in which phosphorylation and dephosphorylation of proteins on tyrosine residues are major events. The involvement of PTPs in such signaling cascades is beginning to emerge from studies on the regulation and mechanisms of action of several representatives of this broad family of proteins.
Similarly to PTKS, PTPs can be classified according to their secondary structure into two broad groups, i.e. cytoplasmic and transmembrane molecules (reviewed in Charbonneau, H., and Tonks, N. K. (1992) Annu. Rev. Cell Biol. 8:463-493; Pot, D. A., and Dixon, J. E. (1992) Biochim. Biophys. Acta 1136:35-43). Transmembrane PTPs have the structural organization of receptors and thus the potential to initiate cellular signaling in response to external stimuli. These molecules are characterized by the presence of a single transmembrane segment and two tandem PTP domains; only two examples of transmembrane PTPs that have single PTP domains are known, HPTP-P (Krueger, N. X., et al., (1990) EMBO J. 9:3241-3252) and DPTP10D (Tian, S.-S., et al., (1991) Cell 67:675-685).
Nonreceptor PTPs display a single catalytic domain and contain, in addition, non-catalytic amino acid sequences which appear to control intracellular localization of the molecules and which may be involved in the determination of substrate specificity (Mauro, L. J., and Dixon, J. E. (1994) TIBS 19:151-155) and have also been suggested to be regulators of PTP activity (Charbonneau, H., and Tonks, N. K. (1992) Annu. Rev. Cell Biol. 8:463-493). PTP1B (Tonks, N. K., et al., (1988) J. Biol. Chem. 263:6731-6737) is localized to the cytosolic face of the endoplasmic reticulum via its C-terminal 35 amino acids (Frangioni, J. V., et al., (1992) Cell 68:545-560). The proteolytic cleavage of PTP1B by the calcium dependent neutral protease calpain occurs upstream from this targeting sequence, and results in the relocation of the enzyme from the endoplasmic reticulum to the cytosol; such relocation is concomitant with a two-fold stimulation of PTP1B enzymatic activity (Frangioni, J. V., et al., (1993) EMBO J. 12:4843-4856). Similarly, the 11 kDa C-terminal domain of T-cell PTP (Cool, D. E., et al., (1989) Proc. Natl. Acad. Sci. (USA) 86:5257-5261) has also been shown to be responsible for enzyme localization and functional regulation (Cool, D. E., et al., (1990) Proc. Natl. Acad. Sci. (USA) 87:7280-7284; Cool, D. E., et al., (1992) Proc. Natl. Acad. Sci. (USA) 89:5422-5426).
PTPs containing SH2 domains have been described including PTP1C (Shen, S.-H., et al., (1991) Nature 352:736-739), also named HCP (Yi, T., et al., (1992) Mol. Cell. Biol. 12:836-846), SHP (Matthews, R. J., et al., (1992) Mol. Cell. Biol 12:2396-2405) or SH-PTP1 (Plutzky, J., et al., (1992) Proc. Natl. Acad. Sci. (USA) 89:1123-1127), and the related phosphatase PTP2C (Ahmad, S., et al., (1993) Proc. Natl. Acad. Sci. (USA) 90:2197-2201), also termed SH-PTP2 (Freeman Jr., R. M., et al., (1992) Proc. Natl. Acad. Sci. (USA) 89:11239-11243), SH-PTP3 (Adachi, M., et al., (1992) FEBS Letters 314:335-339), PTP1D (Vogel, W., et al., (1993) Science 259:1611-1614) or Syp (Feng, G.-S., et al., (1993) Science 259:1607-1611). The Drosophila csk gene product (Perkins, L. A., et al., (1992) Cell 70:225-236) also belongs to this subfamily. PTP1C has been shown to associate via its SH2 domains with ligand-activated c-Kit and CSF-1 receptor PTKs (Yi, T., and Ihle, J. N. (1993) Mol. Cell. Biol. 13:3350-3358; Young, Y.-G., et al., (1992) J. Biol. Chem. 267:23447-23450) but only association with activated CSF-1 receptor is followed by tyrosine phosphorylation of PTP1C. Syp interacts with and is phosphorylated by the ligand activated receptors for epidermal growth factor and platelet-derived growth factor (Feng, G.-S., et al., (1993) Science 259:1607-1611). Syp has also been reported to associate with tyrosine phosphorylated insulin receptor substrate 1 (Kuhne, M. R., et al., (1993) J. Biol. Chem. 268:11479-11481).
Two PTPs have been identified, PTPH1 (Yang, Q., and Tonks, N. K. (1991) Proc. Natl. Acad. Sci. (USA) 88:5949-5953) and PTPase MEG (Gu, M., et al., (1991) Proc. Natl. Acad. Sci. (USA) 88:5867-5871), which contain a region in their respective N-terminal segments with similarity to the cytoskeletal- associated proteins band 4.1 (Conboy, J., et al., (1986) Proc. Natl. Acad. Sci. (USA) 83:9512-9516), ezrin (Gould, K. L., et al., (1989) EMBO J. 8:4133-4142), talin (Rees, D. J. G., et al., (1990) Nature 347:685-689) and radixin (Funayama, N., et al., (1991) J. Cell Biol. 115:1039-1048). The function of proteins of the band 4.1 family appears to be the provision of anchors for cytoskeletal proteins at the inner surface of the plasma membrane (Conboy, J., et al., (1986) Proc. Natl. Acad. Sci. (USA) 83:9512-9516; Gould, K. L., et al., (1989) EMBO J. 8:4133-4142). It has been postulated that PTPH1 and PTPase MEG would, like members of this family, localize at the interface between the plasma membrane and the cytoskeleton and thereby be involved in the modulation of cytoskeletal function (Tonks, N. K., et al., (1991) Cold Spring Harbor Symposia on Quantitative Biology LVI:265-273).
The interest in studying PTKs and PTPs is particularly great in cancer research. For example, approximately one third of the known oncogenes include PTKs (Hunter, T. (1989) In Oncogenes and Molecular Origins of Cancer, R. Weinberg, Ed., Coldspring Harbor Laboratory Press, New York). In addition, the extent of tyrosine phosphorylation closely correlates with the manifestation of the transformed phenotype in cells infected by temperature-sensitive mutants of rous sarcoma virus. (Sefton, B., et al., (1980) Cell 20:807-816) Similarly, Brown-Shirner and colleagues demonstrated that over-expression of PTP1B in 3T3 cells suppressed the transforming potential of oncogenic neu, as measured by focus formation, anchorage-independent growth and tumorigenicity (Brown-Shirner, S., et al., (1992) Cancer Res. 52:478-482). Because they are direct antagonists of PTK activity, the PTPs also may provide an avenue of treatment for cancers caused by excessive PTK activity. Therefore, the isolation, characterization and cloning of various PTPs is an important step in developing, for example, gene therapy to treat PTK oncogene cancers.